The invention relates to radiation monitoring, e.g. to systems for ascertaining intermittently or continuously the radiation level in installations such as a nuclear reactor. Radiation monitoring involves the use of radiation detectors placed at selected locations. Of all basic types of detectors, the present invention pertains to the "on-line" and/or "off-line" type of radioactive gas and/or liquid detector. See for instance--Westinghouse Engineer, January 1972, Vol. 32, pp. 2-8, "Radiation Monitoring System for Nuclear Power Plants" by S. A. Lane, C. Griesaker and T. Hamburger. Also "Radiation Monitoring--An Instrumentation System for Control of Radiation Hazards" by R. Eastman and D. M. Gallagher, IEEE Trans. Nuclear Sci. Vol. NS-12, No. 6, pp. 15-21 (December 1965).
Detection of radioactivity in a gas, or liquid, involves the combination of an on-line, or an off-line, volume of radioactive fluid and a detector of the radiation emitted by such a volume. However, such a detection most of the time is taking place in the presence of a significant background of radiation, which moreover may vary substantially. One of the precautions taken to subtract such background from the measurement has been to make an initial calibration of the detector. The intervening variations in the background level, however, will defeat the purpose. Shielding of the equipment is not satisfactory because effective shielding would be prohibitive in cost and weight.
Therefore, the problem is to obtain a true radioactivity measurement with radioactive fluid while accepting some sort of background information in the derived reading.
A number of approaches have been used in the past to solve this problem, but they all have failed because of serious drawbacks.
A first approach is to measure the background separately, using an auxiliary detector, e.g., a small counter disposed externally. This is not satisfactory because while the auxiliary detector is outside the normal shielding, the background radiation effect on the main monitor, as seen from inside the shielding, is much different from the one detected outside. This is particularly true with gamma rays, which are the radiation encountered in a nuclear reactor plant. Therefore, the outside counter will often give a false indication of the background counts seen by the monitor detector itself. Clearly, under these circumstances the compensation will not be effective.
A second approach consists in purging the detection monitoring system of the radioactive gas or liquid under detection, and measuring the background level separately with the main detector or counter. The main drawback in this case is the resulting interruption of the radiation measurement. Any rapid or continual increases in radioactivity release levels, which occur during these purging periods, will not be detected so that the opportunity for rapid corrective action in the plant may be missed.